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  <div class="section" id="virtual-environments-and-packages">
<span id="tut-venv"></span><h1>12. Virtual Environments and Packages<a class="headerlink" href="#virtual-environments-and-packages" title="Permalink to this headline">¶</a></h1>
<div class="section" id="introduction">
<h2>12.1. Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Python applications will often use packages and modules that don&#8217;t
come as part of the standard library.  Applications will sometimes
need a specific version of a library, because the application may
require that a particular bug has been fixed or the application may be
written using an obsolete version of the library&#8217;s interface.</p>
<p>This means it may not be possible for one Python installation to meet
the requirements of every application.  If application A needs version
1.0 of a particular module but application B needs version 2.0, then
the requirements are in conflict and installing either version 1.0 or 2.0
will leave one application unable to run.</p>
<p>The solution for this problem is to create a <a class="reference internal" href="../glossary.html#term-virtual-environment"><em class="xref std std-term">virtual
environment</em></a> (often shortened to &#8220;virtualenv&#8221;), a self-contained
directory tree that contains a Python installation for a particular
version of Python, plus a number of additional packages.</p>
<p>Different applications can then use different virtual environments.
To resolve the earlier example of conflicting requirements,
application A can have its own virtual environment with version 1.0
installed while application B has another virtualenv with version 2.0.
If application B requires a library be upgraded to version 3.0, this will
not affect application A&#8217;s environment.</p>
</div>
<div class="section" id="creating-virtual-environments">
<h2>12.2. Creating Virtual Environments<a class="headerlink" href="#creating-virtual-environments" title="Permalink to this headline">¶</a></h2>
<p>The script used to create and manage virtual environments is called
<strong class="program">pyvenv</strong>.  <strong class="program">pyvenv</strong> will usually install the most
recent version of Python that you have available; the script is also
installed with a version number, so if you have multiple versions of
Python on your system you can select a specific Python version by
running <tt class="docutils literal"><span class="pre">pyvenv-3.4</span></tt> or whichever version you want.</p>
<p>To create a virtualenv, decide upon a directory
where you want to place it and run <strong class="program">pyvenv</strong> with the
directory path:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">pyvenv</span> <span class="n">tutorial</span><span class="o">-</span><span class="n">env</span>
</pre></div>
</div>
<p>This will create the <tt class="docutils literal"><span class="pre">tutorial-env</span></tt> directory if it doesn&#8217;t exist,
and also create directories inside it containing a copy of the Python
interpreter, the standard library, and various supporting files.</p>
<p>Once you&#8217;ve created a virtual environment, you need to
activate it.</p>
<p>On Windows, run:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="o">/</span><span class="n">Scripts</span><span class="o">/</span><span class="n">activate</span>
</pre></div>
</div>
<p>On Unix or MacOS, run:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">source</span> <span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="o">/</span><span class="nb">bin</span><span class="o">/</span><span class="n">activate</span>
</pre></div>
</div>
<p>(This script is written for the bash shell.  If you use the
<strong class="program">csh</strong> or <strong class="program">fish</strong> shells, there are alternate
<tt class="docutils literal"><span class="pre">activate.csh</span></tt> and <tt class="docutils literal"><span class="pre">activate.fish</span></tt> scripts you should use
instead.)</p>
<p>Activating the virtualenv will change your shell&#8217;s prompt to show what
virtualenv you&#8217;re using, and modify the environment so that running
<tt class="docutils literal"><span class="pre">python</span></tt> will get you that particular version and installation of
Python.  For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">-&gt;</span> <span class="n">source</span> <span class="o">~/</span><span class="n">envs</span><span class="o">/</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="o">/</span><span class="nb">bin</span><span class="o">/</span><span class="n">activate</span>
<span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">python</span>
<span class="n">Python</span> <span class="mf">3.4</span><span class="o">.</span><span class="mi">3</span><span class="o">+</span> <span class="p">(</span><span class="mf">3.4</span><span class="p">:</span><span class="n">c7b9645a6f35</span><span class="o">+</span><span class="p">,</span> <span class="n">May</span> <span class="mi">22</span> <span class="mi">2015</span><span class="p">,</span> <span class="mi">09</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">25</span><span class="p">)</span>
  <span class="o">...</span>
<span class="o">&gt;&gt;&gt;</span> <span class="kn">import</span> <span class="nn">sys</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">sys</span><span class="o">.</span><span class="n">path</span>
<span class="p">[</span><span class="s">&#39;&#39;</span><span class="p">,</span> <span class="s">&#39;/usr/local/lib/python34.zip&#39;</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span>
<span class="s">&#39;~/envs/tutorial-env/lib/python3.4/site-packages&#39;</span><span class="p">]</span>
<span class="o">&gt;&gt;&gt;</span>
</pre></div>
</div>
</div>
<div class="section" id="managing-packages-with-pip">
<h2>12.3. Managing Packages with pip<a class="headerlink" href="#managing-packages-with-pip" title="Permalink to this headline">¶</a></h2>
<p>Once you&#8217;ve activated a virtual environment, you can install, upgrade,
and remove packages using a program called <strong class="program">pip</strong>.  By default
<tt class="docutils literal"><span class="pre">pip</span></tt> will install packages from the Python Package Index,
&lt;<a class="reference external" href="https://pypi.python.org/pypi">https://pypi.python.org/pypi</a>&gt;.  You can browse the Python Package Index
by going to it in your web browser, or you can use <tt class="docutils literal"><span class="pre">pip</span></tt>&#8216;s
limited search feature:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">search</span> <span class="n">astronomy</span>
<span class="n">skyfield</span>               <span class="o">-</span> <span class="n">Elegant</span> <span class="n">astronomy</span> <span class="k">for</span> <span class="n">Python</span>
<span class="n">gary</span>                   <span class="o">-</span> <span class="n">Galactic</span> <span class="n">astronomy</span> <span class="ow">and</span> <span class="n">gravitational</span> <span class="n">dynamics</span><span class="o">.</span>
<span class="n">novas</span>                  <span class="o">-</span> <span class="n">The</span> <span class="n">United</span> <span class="n">States</span> <span class="n">Naval</span> <span class="n">Observatory</span> <span class="n">NOVAS</span> <span class="n">astronomy</span> <span class="n">library</span>
<span class="n">astroobs</span>               <span class="o">-</span> <span class="n">Provides</span> <span class="n">astronomy</span> <span class="n">ephemeris</span> <span class="n">to</span> <span class="n">plan</span> <span class="n">telescope</span> <span class="n">observations</span>
<span class="n">PyAstronomy</span>            <span class="o">-</span> <span class="n">A</span> <span class="n">collection</span> <span class="n">of</span> <span class="n">astronomy</span> <span class="n">related</span> <span class="n">tools</span> <span class="k">for</span> <span class="n">Python</span><span class="o">.</span>
<span class="o">...</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">pip</span></tt> has a number of subcommands: &#8220;search&#8221;, &#8220;install&#8221;, &#8220;uninstall&#8221;,
&#8220;freeze&#8221;, etc.  (Consult the <a class="reference internal" href="../installing/index.html#installing-index"><em>Installing Python Modules</em></a> guide for
complete documentation for <tt class="docutils literal"><span class="pre">pip</span></tt>.)</p>
<p>You can install the latest version of a package by specifying a package&#8217;s name:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">install</span> <span class="n">novas</span>
<span class="n">Collecting</span> <span class="n">novas</span>
  <span class="n">Downloading</span> <span class="n">novas</span><span class="o">-</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span><span class="o">.</span><span class="n">tar</span><span class="o">.</span><span class="n">gz</span> <span class="p">(</span><span class="mi">136</span><span class="n">kB</span><span class="p">)</span>
<span class="n">Installing</span> <span class="n">collected</span> <span class="n">packages</span><span class="p">:</span> <span class="n">novas</span>
  <span class="n">Running</span> <span class="n">setup</span><span class="o">.</span><span class="n">py</span> <span class="n">install</span> <span class="k">for</span> <span class="n">novas</span>
<span class="n">Successfully</span> <span class="n">installed</span> <span class="n">novas</span><span class="o">-</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span>
</pre></div>
</div>
<p>You can also install a specific version of a package by giving the
package name  followed by <tt class="docutils literal"><span class="pre">==</span></tt> and the version number:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">install</span> <span class="n">requests</span><span class="o">==</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span>
<span class="n">Collecting</span> <span class="n">requests</span><span class="o">==</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span>
  <span class="n">Using</span> <span class="n">cached</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span><span class="o">-</span><span class="n">py2</span><span class="o">.</span><span class="n">py3</span><span class="o">-</span><span class="n">none</span><span class="o">-</span><span class="nb">any</span><span class="o">.</span><span class="n">whl</span>
<span class="n">Installing</span> <span class="n">collected</span> <span class="n">packages</span><span class="p">:</span> <span class="n">requests</span>
<span class="n">Successfully</span> <span class="n">installed</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span>
</pre></div>
</div>
<p>If you re-run this command, <tt class="docutils literal"><span class="pre">pip</span></tt> will notice that the requested
version is already installed and do nothing.  You can supply a
different version number to get that version, or you can run <tt class="docutils literal"><span class="pre">pip</span>
<span class="pre">install</span> <span class="pre">--upgrade</span></tt> to upgrade the package to the latest version:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">install</span> <span class="o">--</span><span class="n">upgrade</span> <span class="n">requests</span>
<span class="n">Collecting</span> <span class="n">requests</span>
<span class="n">Installing</span> <span class="n">collected</span> <span class="n">packages</span><span class="p">:</span> <span class="n">requests</span>
  <span class="n">Found</span> <span class="n">existing</span> <span class="n">installation</span><span class="p">:</span> <span class="n">requests</span> <span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span>
    <span class="n">Uninstalling</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span><span class="p">:</span>
      <span class="n">Successfully</span> <span class="n">uninstalled</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.6</span><span class="o">.</span><span class="mi">0</span>
<span class="n">Successfully</span> <span class="n">installed</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">pip</span> <span class="pre">uninstall</span></tt> followed by one or more package names will remove the
packages from the virtual environment.</p>
<p><tt class="docutils literal"><span class="pre">pip</span> <span class="pre">show</span></tt> will display information about a particular package:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">show</span> <span class="n">requests</span>
<span class="o">---</span>
<span class="n">Metadata</span><span class="o">-</span><span class="n">Version</span><span class="p">:</span> <span class="mf">2.0</span>
<span class="n">Name</span><span class="p">:</span> <span class="n">requests</span>
<span class="n">Version</span><span class="p">:</span> <span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span>
<span class="n">Summary</span><span class="p">:</span> <span class="n">Python</span> <span class="n">HTTP</span> <span class="k">for</span> <span class="n">Humans</span><span class="o">.</span>
<span class="n">Home</span><span class="o">-</span><span class="n">page</span><span class="p">:</span> <span class="n">http</span><span class="p">:</span><span class="o">//</span><span class="n">python</span><span class="o">-</span><span class="n">requests</span><span class="o">.</span><span class="n">org</span>
<span class="n">Author</span><span class="p">:</span> <span class="n">Kenneth</span> <span class="n">Reitz</span>
<span class="n">Author</span><span class="o">-</span><span class="n">email</span><span class="p">:</span> <span class="n">me</span><span class="nd">@kennethreitz</span><span class="o">.</span><span class="n">com</span>
<span class="n">License</span><span class="p">:</span> <span class="n">Apache</span> <span class="mf">2.0</span>
<span class="n">Location</span><span class="p">:</span> <span class="o">/</span><span class="n">Users</span><span class="o">/</span><span class="n">akuchling</span><span class="o">/</span><span class="n">envs</span><span class="o">/</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="o">/</span><span class="n">lib</span><span class="o">/</span><span class="n">python3</span><span class="o">.</span><span class="mi">4</span><span class="o">/</span><span class="n">site</span><span class="o">-</span><span class="n">packages</span>
<span class="n">Requires</span><span class="p">:</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">pip</span> <span class="pre">list</span></tt> will display all of the packages installed in the virtual
environment:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">pip</span> <span class="nb">list</span>
<span class="n">novas</span> <span class="p">(</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span><span class="p">)</span>
<span class="n">numpy</span> <span class="p">(</span><span class="mf">1.9</span><span class="o">.</span><span class="mi">2</span><span class="p">)</span>
<span class="n">pip</span> <span class="p">(</span><span class="mf">7.0</span><span class="o">.</span><span class="mi">3</span><span class="p">)</span>
<span class="n">requests</span> <span class="p">(</span><span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span><span class="p">)</span>
<span class="n">setuptools</span> <span class="p">(</span><span class="mf">16.0</span><span class="p">)</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">pip</span> <span class="pre">freeze</span></tt> will produce a similar list of the installed packages,
but the output uses the format that <tt class="docutils literal"><span class="pre">pip</span> <span class="pre">install</span></tt> expects.
A common convention is to put this list in a <tt class="docutils literal"><span class="pre">requirements.txt</span></tt> file:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">freeze</span> <span class="o">&gt;</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span>
<span class="p">(</span><span class="n">tutorial</span><span class="o">-</span><span class="n">env</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">cat</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span>
<span class="n">novas</span><span class="o">==</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span>
<span class="n">numpy</span><span class="o">==</span><span class="mf">1.9</span><span class="o">.</span><span class="mi">2</span>
<span class="n">requests</span><span class="o">==</span><span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">requirements.txt</span></tt> can then be committed to version control and
shipped as part of an application.  Users can then install all the
necessary packages with <tt class="docutils literal"><span class="pre">install</span> <span class="pre">-r</span></tt>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">-&gt;</span> <span class="n">pip</span> <span class="n">install</span> <span class="o">-</span><span class="n">r</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span>
<span class="n">Collecting</span> <span class="n">novas</span><span class="o">==</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span> <span class="p">(</span><span class="kn">from</span> <span class="o">-</span><span class="n">r</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span> <span class="p">(</span><span class="n">line</span> <span class="mi">1</span><span class="p">))</span>
  <span class="o">...</span>
<span class="n">Collecting</span> <span class="n">numpy</span><span class="o">==</span><span class="mf">1.9</span><span class="o">.</span><span class="mi">2</span> <span class="p">(</span><span class="kn">from</span> <span class="o">-</span><span class="n">r</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span> <span class="p">(</span><span class="n">line</span> <span class="mi">2</span><span class="p">))</span>
  <span class="o">...</span>
<span class="n">Collecting</span> <span class="n">requests</span><span class="o">==</span><span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span> <span class="p">(</span><span class="kn">from</span> <span class="o">-</span><span class="n">r</span> <span class="n">requirements</span><span class="o">.</span><span class="n">txt</span> <span class="p">(</span><span class="n">line</span> <span class="mi">3</span><span class="p">))</span>
  <span class="o">...</span>
<span class="n">Installing</span> <span class="n">collected</span> <span class="n">packages</span><span class="p">:</span> <span class="n">novas</span><span class="p">,</span> <span class="n">numpy</span><span class="p">,</span> <span class="n">requests</span>
  <span class="n">Running</span> <span class="n">setup</span><span class="o">.</span><span class="n">py</span> <span class="n">install</span> <span class="k">for</span> <span class="n">novas</span>
<span class="n">Successfully</span> <span class="n">installed</span> <span class="n">novas</span><span class="o">-</span><span class="mf">3.1</span><span class="o">.</span><span class="mf">1.3</span> <span class="n">numpy</span><span class="o">-</span><span class="mf">1.9</span><span class="o">.</span><span class="mi">2</span> <span class="n">requests</span><span class="o">-</span><span class="mf">2.7</span><span class="o">.</span><span class="mi">0</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">pip</span></tt> has many more options.  Consult the <a class="reference internal" href="../installing/index.html#installing-index"><em>Installing Python Modules</em></a>
guide for complete documentation for <tt class="docutils literal"><span class="pre">pip</span></tt>.  When you&#8217;ve written
a package and want to make it available on the Python Package Index,
consult the <a class="reference internal" href="../distributing/index.html#distributing-index"><em>Distributing Python Modules</em></a> guide.</p>
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